The present invention relates in general to rotatable heat exchangers, sometimes known as heat recovery wheels, and, in particular, it relates to a novel mounting structure for such heat recovery wheels.
In gas turbines, furnaces, and other combustion chambers it is desirable to preheat incoming combustion gases with heat extracted from exhaust gases, thereby increasing combustion efficiency. Rotary heat exchangers or heat recovery wheels are particularly desirable for this purpose. Heat recovery wheels are generally mounted for rotation about a central axis. The wheel first rotates through the exhaust gas stream. The wheel has apertures extending therethrough in the axial direction. Gases flow axially through the apertures and heat is extracted from the exhaust gas stream and absorbed by the wheel. As the wheel further rotates through the incoming gas stream, this heat is given up by the wheel to the incoming gas.
Most conventionally, heat recovery wheels are centrally supported and driven by a torque applied to the hub thereof. However, it is also known to drive such wheels from the periphery. In either case, it is particularly important that the rotational stability of the wheel be maintained, i.e. it is important that the wheel remain centered about its axis of rotation as the temperatures to which it is subjected vary. When the axis of rotation varies, forces exerted upon the wheel may cause its breakage. Also, in the case of peripherally driven wheels, the wheel when rotating eccentrically may lose contact with its driving means.
While heat recovery wheels may be made from metal, ceramic materials are preferred because ceramic materials can have a low coefficient of thermal expansion and a high resistance to oxidation and corrosion. However, even though ceramics are chosen for the body of such wheels, metals are still employed for the driver hub. Since the coefficient of expansion of the metal hub differs from the coefficient of expansion of the ceramic, it is generally necessary to securely mount the wheel to its hub while permitting differential thermal expansion of the hub in the radial direction with respect to the wheel. U.S. Pat. No. 3,774,675--Yoshiro discloses such a mounting arrangement. In the Yoshiro patent, pads are provided which are radially expandable within the bore of a ceramic heat recovery wheel. A plurality of spring means are provided which bias the pads radially outwardly. One problem associated with the aforementioned Yoshiro design is that with that approach, since a plurality of spring means are employed, it is impossible to select springs having exactly equal spring constants. Since the springs have varying spring constants, the heat recovery wheel of Yoshiro is very difficult to center about its axis of rotation.
It would be desirable to provide a mounting device for a ceramic heat recovery wheel which is self-centering. It is an object of the present invention to provide a mounting device for a heat recovery wheel which is self-centering. It is a further object of the present invention to provide a mounting device which is self-centering and yet which permits the differential radial expansion of a metal hub with a ceramic wheel.